A chemically modified antibody in which a reactive group is juxtaposed to the binding site for a hapten may assume enzyme-like properties. The binding to antibody may accelerate the reaction of a homologous or hapten analog with the covalently linked species. We propose to use a chelated zinc species or a flavin derivative, as examples of enzymic cofactors, for the expression of hydrolysis or redox behavior respectively, in anti hapten antibodies. A haloacetyl derivative of the cofactor may be regioselectively incorporated into an antibody which is chemically thiolated near the combining region. The required thiol group is introduced in the correct position by an affinity labeling scheme using the hapten to deliver a photoreactive, disulfide linked appendage. Photolysis and disulfide reduction exposes the covalently linked thiol appendage. The non-covalently bound hapten may be removed and the chemically modified antibody that remains should serve for the production of enzyme-like substances. Such modified antibodies might also be useful for the ligation of probes for the investigation of antigen structure. The production of artificial enzymes is a widespread effort which is being approached from many directions. Advances in this area have great theoretical significance as well as practical implications. Synthetic or semisynthetic enzymes may eventually be useful for the treatment of disease, as diagnostic reagents, or as industrial catalysts. The use of immunoglobulins as starting materials for semisynthetic enzymes is a new approach, advanced here for the first time.